Silver Miller — America’s leading cryptocurrency investor law firm — announced it has filed lawsuits against AT&T and T-Mobile on behalf of crypto asset holders who were allegedly victims of “SIM swapping.”

The suit alleges that by leaving holes in their security protocols, and failing to train and monitor employees adequately, the phone providers assisted criminals in remotely taking over smartphone SIM cards, allowing them to access financial records and account information to steal cryptocurrency and other assets.

The complaint explains how one of Silver Miller’s clients — an AT&T account holder — was robbed of over $621,000 in cryptocurrency during an SIM swap, even after the mobile carrier had supposedly boosted security on the user’s account after an earlier attempted theft. In addition, other Silver Miller clients, who used T-Mobile, were robbed of $400,000 and $250,000.

“‘SIM swapping’ or ‘SIM hijacking’ is a growing crime in the telecommunications world that, in a few moments’ time, can allow a thief to steal millions of dollars of an unsuspecting victim’s assets with little more than a persuasive plea for assistance, a willing telecommunications carrier representative and an electronic impersonation of the victim,” according to Silver Miller in a press release.

Last year, Myriad Connect, a mobile technology specialist, announced the launch of a new service designed to counter SIM swap fraud, which is also costing banks and mobile network providers millions of dollars.

Myriad Connect said its service can help reduce SIM swap by providing a real-time check on the SIM, which cannot be tampered with via compromised third parties within an operator or bank. Using Unstructured Supplementary Service Data (USSD) authentication, no data is held with any third party, providing a more secure service. Myriad said a clear audit trail is also established, where the user’s identity is verified by a party external to the transaction. This results in a technology that will greatly enhance the security of transactions vulnerable to SIM swap fraud.